Short vs long scope in regard to exit pupil and magnification

[Sonmalul]

There is something that makes me think practicality.  

We can all agree that a long focal length scope is better suited for planetary use than its short length brother which would be best suited for dso viewing or guiding/photo. 

We can take those two (refractors) as an example: 90/900 and his brother, a 90/500 (and we will be using Plossl EPs)

Exit pupil is an important factor in viewing comfort. 

Exit pupils are dependent on the scope's focal length, the EP's fl and the main lens diameter.  

For example, to acheive 100x in the 90/500 we will need a 5mm eyepiece and to acheive the same mag in the 90/900 we will need a 9mm eyepiece. 

The exit pupils of both scopes in those configurations will be 90/100=0.9mm 

The theoretical magnification limit for both scopes lies at about 180x. 

It is true that to reach this 180x you will need a 5mm EP in the 90/900 and a "2.8"mm EP in the 90/500 and those will probably make for an eyelash brushing eye relief... But looking at exit pupils up to 100x, those scopes do not seem to complement one another, but rather replace each other.

So, ignoring abberations, and not taking into account eye relief so much, wouldn't the faster scope be almost as practical and easy on the eyes for high-ish magnifications as the slower, heavier, harder to mount in the wind scope?

just my evening neurons buzzing...

Edited May 24, 2020 by Sonmalul

[Shaun VS]

I think that the one factor that needs to be included is lens quality. For achromatic lenses the longer focal ratio will exhibit less false colour and hence finer detail. With apos the colours are brought together better, thus giving sharper images. The shorter focal ratio is then less or not an issue.

 

[johninderby]

One very important thing in planetary viewing is depth of field. Longer focal length scopes have greater depth of field which means that when observing a planet more of the surface or indeed the whole planet is in focus whereas with a short focal length less of the planet is in sharp focus.

Edited May 26, 2020 by johninderby

[Pixies]

17 hours ago, johninderby said:

One very important thing in planetary viewing is depth of field. Longer focal length scopes have greater depth of field which means that when observing a planet more of the surface or indeed the whole planet is in focus whereas with a short focal length less of the planet is in sharp focus.

Hi John,

This puzzles me slightly. If the object is at infinity (practically), then won't the depth of field be infinite too?

I can understand that the depth of focus at the eye could be shorter - so more precise focussing is required to get it in focus. But I can't see how part of a planet could be in focus, and another not? Sorry if I have misunderstood.

[johninderby]

The faster or shorter focal length telescope has a smaller range of focus so on something like a planet the outer edge of the planet may focus at a slightly different setting.

Edited May 27, 2020 by johninderby

[Pixies]

Yes, but at infinity, the depth of field is effectively infinite too, irrespective of a few inches in aperture? The depth of focus is much tighter, so you have to be more precise to get a focus at the EP. But I can't see how parts of a planet require different focussing when it is at such a great distance?

 

 

[johninderby]

In the real world you can see the difference when you look at a planet with a long focal length scope vs a short focal length. 

[andrew s]

Are two things at play here. Depth of field refers to how well or easily you can focus. Even the moon is so far away relatively that the whole disc would be in focus if the telescope field were flat.

However, most refractors naturally have a curve focal plane. The shorter the focal length the greater the curvature. Maybe this is the reason for preferring the longer focal length. When young your eyes can easily accomodate for this but with age less so.

Regards Andrew 

Edited May 27, 2020 by andrew s

[johninderby]

It’s more trying to explain something that I have noticed in real world observing. I wonder if everyone sees the same effect or do differences in eyesight play a part?

[andrew s]

3 minutes ago, johninderby said:

It’s more trying to explain something that I have noticed in real world observing. I wonder if everyone sees the same effect or do differences in eyesight play a part?

Science applies to the real world of observation . Yes your eyes will have a major effect, as I mentioned accomodation but also visual accruity,  dark adaption, practice and skill

Regards Andrew 

[Louis D]

On 24/05/2020 at 14:29, Sonmalul said:

So, ignoring abberations, and not taking into account eye relief so much, wouldn't the faster scope be almost as practical and easy on the eyes for high-ish magnifications as the slower, heavier, harder to mount in the wind scope?

Yes, you've basically derived why fast-ish ED and APO refractors are popular, and why big Dobs have gotten progressively faster and thus squatter.  It also accounts for the popularity of specialty, short focal length eyepieces like the Vixen HRs and Tak TOEs to achieve high magnifications and small exit pupils while maintaining ease of use and high image quality.

[Sonmalul]

11 hours ago, Louis D said:

Yes, you've basically derived why fast-ish ED and APO refractors are popular, and why big Dobs have gotten progressively faster and thus squatter.

Then why do they still produce scopes in the f10+ range alongside their identical aperture and mechanics, faster brothers?

what's the catch?

Seems to me that progress in the refractor world look like going from something lile this

https://upload.wikimedia.org/wikipedia/commons/9/9a/Houghton_Typ_620.73.451_-_Johannes_Hevelius%2C_Machinae_coelestis%2C_1673.jpg

to stuff like this

https://cdn2.skiesunlimited.com/images/D/09947_Orion_alt04.jpg

😜

[Sonmalul]

On 26/05/2020 at 20:02, johninderby said:

One very important thing in planetary viewing is depth of field. 

This is indeed the only reason I could think of as a reason to manufacturing many lengths of the same aperture frac. 

Other than that, magnification and exit pupil are just complementary mathematical relations. Like horsepower, torque and rpm. 

[Sonmalul]

20 hours ago, johninderby said:

It’s more trying to explain something that I have noticed in real world observing. I wonder if everyone sees the same effect or do differences in eyesight play a part?

I like to play tricks on my eyes with binoculars to see if my eyes can reach focus when the instrument in marginaly out of focus. 

They struggle a bit, it feels like havy lifting done by my eye's lens but I was amazed how off can the focus be and the eye still be able to adapt, albeit after a bit of "flex". 

This range of natural focus (for me at least) is quite large and i do my focusing by choosing the mechanical middle between the in/out of focus points where my eyes cannot compensate at all.  So back-forth back-forth...something like divide et impera for the focuser tube 😋

Differences in depth of field have not been too striking to my eyes through telescopes as they are in an f2 photo lens for example. 

I feel like we have two focusing systems: our hands on the focuser knobs and our eyes which do the fine tuning for us. So focus in focus :)

[johninderby]

And there is that hard to quantify thing of a long focal length achro can be enjoyable to use. There is just something about a top quality achro. Many think they are outdated and who would want one instead of a “ better” APO? However look through one and you can get hooked. 

 

 

[John]

29 minutes ago, Sonmalul said:

Then why do they still produce scopes in the f10+ range alongside their identical aperture and mechanics, faster brothers?

what's the catch?

Seems to me that progress in the refractor world look like going from something lile this

https://upload.wikimedia.org/wikipedia/commons/9/9a/Houghton_Typ_620.73.451_-_Johannes_Hevelius%2C_Machinae_coelestis%2C_1673.jpg

to stuff like this

https://cdn2.skiesunlimited.com/images/D/09947_Orion_alt04.jpg

😜

A few years ago the long achromatic refractor was indeed dying out.

Over the past few years, influenced by a few articles and publications by authors such as Neil English, an interest in these traditional designs has reignited among some visual observers. So the vintage long refractors became sought after and some newly manufactured units have emerged, notably the 102mm F/11 achromat which Lyra Optics imported from the far east a few years back, Richard Day's composite creations (as showcased by @johninderby above, Moonraker Telescopes and there are a few more options now.

If these designs are appreciated, and enjoyed and people are prepared to spend their hard earned money on them, it's good that manufacturers have responded. Choice is a good thing I think

[johninderby]

BTW Big Red is my homebuilt scope based on the Carton 100mm f/13 optics.

 

Edited May 28, 2020 by johninderby

[andrew s]

Fashion plays a role even in Astronomy. One or other aspect takes over, like hem length in dresses, being highlighted over some other factor. Short easy to mount and house v long and more difficult. 

As technology opens up other options they rise in popularity to fall back as the wave passes.

We are fortunate to have a wide range of options over an unimaginable price range.

Regards Andrew 

[Louis D]

5 hours ago, Sonmalul said:

Then why do they still produce scopes in the f10+ range alongside their identical aperture and mechanics, faster brothers?

what's the catch?

Cost and level of correction.  Since there are no regulations on minimum acceptable levels of optical correction and mechanical refinement for astronomical telescopes and mounts, manufacturers make everything from $25 toy telescopes with terrible optics and mechanics to multi-hundred thousand dollar scopes with excellent correction and superb mechanics.  Imagine if there were no street regulations on cars.  Manufacturers would be putting out dune-buggy type kits for street use powered by lawn mower engines side by side with luxury cars:

vs.

Just as you can't make a Bugatti for the cost of a go kart, you can't make a 12" APO for the cost of a 50mm department store telescope.

[Sonmalul]

On 28/05/2020 at 11:58, johninderby said:

BTW Big Red is my homebuilt scope based on the Carton 100mm f/13 optics.

Big Red is a thing of beauty and mounted on that wooden tripod it looks like it is going to last a lifetime...and your children's lifetime also :)

[Sonmalul]

On 28/05/2020 at 17:23, Louis D said:

Just as you can't make a Bugatti for the cost of a go kart, you can't make a 12" APO for the cost of a 50mm department store telescope.

That would make it a Bugatti Mowron :)

[Sonmalul]

On 28/05/2020 at 12:04, andrew s said:

Fashion plays a role even in Astronomy. 

I always feared that if I say here I want to buy a very specific brand and type of scope I would be ridiculed over the roof. 

But truth ne told, we keep pur scopes mostly indoors and they become part of our furniture in some way. 

I enjoy looking at things even when they are not used for their intended purpose, just to admire the craftmanship. 

[Louis D]

2 hours ago, Sonmalul said:

But truth ne told, we keep pur scopes mostly indoors and they become part of our furniture in some way. 

I enjoy looking at things even when they are not used for their intended purpose, just to admire the craftmanship. 

I'm guessing you don't have little kids and pets that tend to accidentally knock things over or otherwise damage things.  All of my astro gear sheltered in the backs of a couple of coat closets to protect them during my child rearing years.  Even though I'm an empty nester now, I continue to store them there out of habit.

[andrew s]

I read something that maybe of relevance in Telescope Optics by Rutten & van Venrooij.

They were commenting on field curvature of eyepieces and how difficult it is to remove without introducing astigmatism.  The also commented on the fact that the f number the eyepiece has to work at is the same as that of the telescope and that they had to work at much higher off axis angles. All in all this makes aberration control difficult especially field curvature. It also gets more challenging at shorter focal lengths.

Thay also pointed out that the eye has to accommodate to account for the field curvature of the eyepiece and that this gets harder as you get older.

Thus all factors point to a longer focal ratio, longer focal length telescope paired with a longer focal length eyepiece being easier on the eye.

Regards Andrew